Extensible boom

ABSTRACT

In an extensible boom of the type having telescopically extensible boom members, a buckling-preventing method for hydraulic cylinder unit employed in the boom wherein an intermediate position of an optionally extended piston-rod of said cylinder unit is always supported with a buckling-preventing device having legs slidably or rotatably engaged with inner circumference of a boom member. The present invention further includes driving means for said buckling-preventing device which moves the device to said intermediate position of the piston-rod in connection with the operation of said cylinder unit.

United States Patent [191 Sakamoto et al.

[ 1 Sept. 17, 1974 EXTENSIBLE BOOM Inventors: Teruya Sakamoto, 2482-70,

Midori-cho, Sakaide', Takemi Nishimura, 230-14, Nonowe, Havikino; Osamu Takarada, 1475 Omachi, Muro-cho, Kita-Gun, Kagawa, all of Japan Filed: Dec. 26, 1972 Appl. No.: 318,608

Related US. Application Data Division of Ser. No.' 80,913, Oct, 15, 1970.

Foreign Application Priority Data Jan. 31, 1970 Japan 45-8718 Feb. 2, 1970 Japan 45-9452 US. Cl. 212/144, 92/165 R Int. Cl. B66c 23/62 Field of Search 1. 212/144; 92/165 R;

References Cited UNITED STATES PATENTS Munschaucr 92/165 R X Crawley et al. 92/165 R Thompson 212/144 X Primary Examiner-Robett B. Reeves Assistant ExaminerHadd Lane Attorney, Agent, or FirmOlson, Trexler. Wolters, Bushnell & Fosse, Ltd.

13 Claims, 28 Drawing Figures PAIENIEDSEP 1 mm 3.836.01 1

SHEET 2 0F 7 PAIENTEDSEPI 11924 SHEET 3 BF 7 PAIENTEUSEPI W4 7 3,836.01 1

SHEET s 0F 7 PAIENTEDSEH 71914 SHEET 5 OF 7 Q Q Q m PAIENIE SEP 1 mm SHEET 6 BF 7 1 EXTENSIBLE BOOM REFERENCE TO RELATED APPLICATION This application is a division of application Ser. No. 80,913, filed Oct. 15, 1970.

SUMMARY OF TI-IE INVENTION This invention relates to an extensible boom of the type having telescopically extensible boom members, and more particularly to a buckling-preventing method for hydraulic cylinder unit employed in the boom of the aforementioned type and an apparatus therefor.

DISTINCTIONS OVER THE PRIOR ART AND OBJECTS In an extensible boom for a crane comprising an inner boom member telescopically inserted into an outer boom member, there is arranged a hydraulic cylinder unit for extending and contracting the boom. That is, in the extensible boom, the piston-rod of said cylinder unit is connected at the free end thereof to the outer or inner boom member, and the cylinder of said unit is connected at a suitable portion thereof to the other boom member, so that the extensible boom is extended or contracted as the hydraulic cylinder unit is extended or contracted. A compressive stress applied to the extensible boom is supported by the hydraulic cylinder unit, so that extremely high compressive stress is applied to the unit during loading and unloading works with a crane.

When the extensible boom or the hydraulic cylinder unit is extended, the resistivity against buckling of the cylinder unit, especially the piston-rod of said unit, namely the resistivity against the breaking due to buckling stress applied to the extended piston-rod, is decreased with the increase in extension of the boom, and

more exactly of the span of extended piston-rod determined with the length of each boom member. In other words, the longer the span is, the higher resistivity against buckling is required.

On the other hand, a crane with a high lift is required recently, so that each boom member of an extensible boom for a crane becomes very long whereby a longer piston-rod of a hydraulic cylinder unit becomes required for extending and contracting such a long boom member.

When the span of an extended piston-rod becomes longer, a piston-rod with a larger diameter is usually used for giving a required strength against buckling to the same. However, when the diameter of the pistonrod is increased, then the hydraulic cylinder unit becomes large-scaled and is increased in weight and in cost. There have never been proposed methods or apparatus for avoiding these defects.

Accordingly, the primary object of the present invention is to provide a novel buckling-preventing method for hydraulic cylinder unit employed in an extensible boom by which above defects are fully avoided and which guarantees the exclusion of the bending of extended piston-rod due to buckling stress applied to the rod.

Another object of the present invention is to provide a buckling-preventing method for hydraulic cylinder unit which utilizes boom members skillfully so as to achieve the primary object in a simple manner.

The above objects are attained according to the present invention by providing a buckling-preventing device which comprises a portion slidably received with the piston-rod and legs slidably or rotatably engaged with the inner circumference of the boom, and by moving said device into a predetermined intermediate position between the free end of the piston-rod and the end of the cylinder in connection with the extending and contracting operation of hydraulic cylinder unit.

The present invention further aims at the provision of driving method and means for the aforementioned buckling-preventing device which allow to move the device into said predetermined position in a desirable manner.

BRIEF DESCRIPTION OF THE DRAWINGS The present invention and its advantages will become more readily apparent as the specification is considered in conjunction with the accompanying drawings in which:

FIG. 1 is a sectional side view of an embodiment of two-stage extensible boom according to the present invention showing a contracted state and taken along line 11 of FIG. 3;

FIG. 2 is a sectional side view of the extensible boom shown in FIG. 1 showing an extended state;

FIG. 3 is a sectional view taken along line 3-3 of FIG. 2;

FIG. 4 is a sectional side view of another embodiment of a two-stage extensible boom according to the present invention;

FIG. 5 is a sectional side view of still another embodiment of a three-stage extensible boom according to the present invention;

FIG. 6 is a sectional side view of a further embodiment of a two-stage extensible boom according to the present invention;

FIG. 7 is a sectional side view of a still further embodiment of a two-stage extensible boom according to the present invention;

FIG. 8 is a sectional side view of another embodiment of a three-stage extensible boom according to the present invention;

FIG. 9 is a sectional side view of still another embodiment of a two-stage extensible boom according to the present invention;

FIG. 10 is a sectional view taken along line 10l0 of FIG. 9;

FIG. 11 is a sectional side view of a further embodiment of a two-stage extensible boom according to the present invention;

FIG. 12 is a side view of another embodiment of employed rollers according to the present invention;

FIG. 13 is a still further embodiment of a three-stage extensible boom according to the present invention;

FIG. 14 is a side view showing the relation between plate members and rollers;

FIG. 15 is a sectional side view of another embodiment of a two-stage extensible boom according to the present invention;

FIG. 16 is an enlarged perspective view of the drum shown in FIG. 15;

FIG. 17 is a sectional side view of still another embodiment of a two-stage extensible boom according to the present invention;

FIG. 18 is a sectional view of the extensible boom taken along line 18-18 of FIG. 17;

FIG. 19 is a sectional side view of a further embodiment of a two-stage extensible boom according to the present invention;

FIG. 20 is a sectional side view of a still further embodiment of a two-stage extensible boom according to the present invention;

FIG. 21 is a sectional side view of another embodiment of a two-stage extensible boom according to the present invention;

FIG. 22 is an enlarged sectional view taken along line 2222 of FIG. 21;

FIG. 23 is a sectional view taken along line 2323 of FIG. 22;

FIG. 24 is a sectional side view of a still further embodiment of a two-stage extensible boom according to the present invention showing an extended state;

FIG. 25 is a sectional side view of the extensible boom shown in FIG. 24;

FIG. 26 is an enlarged sectional side view, partly cutaway, of a part of the extensible boom shown in FIG. 25;

FIG. 27 is a sectional view taken along line 27-27 of FIG. 26; and

FIG. 28 is a rear view of the part shown in FIG. 26.

DETAILED DESCRIPTION Referring now to the drawings, wherein like numerals designate like parts throughout the various figures thereof, one embodiment of an extensible boom, constructed in accordance with the present invention is designated by the reference numeral 1 and illustrated in FIGS. 1, 2 and 3.

The extensible boom 1 includes an outer boom member 2 telescopically receiving an inner boom member 3 therein and an inverted hydraulic cylinder unit 4, the piston-rod 5 of which is pivotally connected at its free end portion to the lower end portion of the outer boom member 2 by suitable means such as pin 6 and the cylinder 7 of which is pivotally connected at one end portion to the lower end portion of the inner boom member 3, as generally indicated by the reference numeral 8.

In accordance with the present invention, a bucklepreventing device 9 is provided within the boom 1 and comprises supporting legs 10 with shoes 10a slidably engaged with the inner circumference of the outer boom member 2 and a boss 11 slidably mounted on the piston rod 5. The shoes 10a may be interchanged with rollers 10b shown in FIG. 9. v

The device 9 further includes two pulleys l2 and 13 the shafts of which are secured to legs 10. One of the pulleys 12 trains or carries a wire rope 14 which is suitably fastened at one end to the lower end of the inner boom member 3 at 15, the rope extending around the pulley 12 in U-letter-shaped manner and being suitably fastened at the other end to the upper end of the outer boom member 2 at 16. In like fashion, pulley 13 trains or carries a wire rope 17 which is fixed at one end to the lower end of the inner boom member 3 at 18, and which is turned in a U-letter-shaped manner successively at a pulley l9 rotatably mounted onto the upper end of the outer boom member 2, a pulley 20 rotatably mounted onto the lower end of the boom member 2 and the pulley 13, the rope being fixed at the other end to the lower end of the outer boom member 2 at 21.

With the buckle-preventing device connected to extensible boom 1 in the aforementioned manner, wire rope 14 causes boss 11 and supporting legs 10 to move forwardly or away from the pin 6 as cylinder 7 moves the boom to its extended position. More particularly, the incremental movement of boss 11 is one-half that of piston 7, so that the boss always is maintained equivalent distances from the cylinder and the free end of the piston-rod 5. As illustrated in FIG. 2, when cylinder 7 has moved a distance L, boss 11 has moved a distance one-half L. In like manner, boss 11 is moved downwardly or towards pin 6 by the wire rope 17 when the extensible boom 1 is contracted, the boss continuously being maintained equivalent distances from the cylinder and pin 6.

It is to be understood that, for sake of clarity, the pulleys 12 and 13 have been illustrated as being mounted on individual shafts which extend axially along different lines and do not lie on a same line as illustrated in the embodiment shown in FIGS. 1, 2 and 3. For a smooth operation it is preferable to mount both of the pulleys on a common shaft or on respective shafts lying on a common line.

In addition, although an inverted hydraulic cylinder unit 4 and two-stage boom have been shown, it is to be understood that the present invention can be applied to a boom with a normally postured hydraulic cylinder unit (see FIG. 4) and also to a multistage extensible boom having more than three boom members (see FIG. 5). In FIG. 5, numeral 4 designates an additional hydraulic cylinder unit for a third boom member, numeral 9 designates an additional buckle-preventing device, and 14 and 17 designate wire ropes for moving the additional device 9 in the three stage extensible boom in connection with the extending and contracting operations of the cylinder unit 4 respectively.

Each of the embodiments of the present invention shown in FIGS. 1 through 5 provides driving means for buckle-preventing device herein said device is moved by a wire rope which is fixed at one end to a suitable portion of a boom member carrying a cylinder of a hydraulic cylinder unit, is trained over a pulley provided in the buckle-preventing device in such a manner that said rope is turned at said pulley in U-leeter-shaped manner, and is fixed at the other end to an end of a boom member carrying a piston-rod of said cylinder unit, at which end the latter boom member is integrated with the former boom member, and a wire rope, which is fixed at one end to said integrated end of the former boom member, is trained over two pulleys rotatably mounted onto both ends of the latter boom member and another pulley provided in the buckle-preventing device successively in such a manner that said latter rope is turned at each of said latter three pulleys in U letter-shaped manner, and is fixed at the other end to the non-integrated end of the latter boom member.

Turning to FIG. 6, a further embodiment of the present invention is illustrated. In this embodiment, four equally circumferentially spaced elongated racks 22 (only two of which are shown) are secured to the external sides of cylinder 7 and extend parallel to the extending and contracting direction of an extensible boom 1. As illustrated in FIG. 6, the racks 22 extend from the right end of the cylinder and terminate approximately in the middle thereof. In addition, four elongated racks 23 are provided on the inner surface of the inner boom member 3 in confronting face-to-face relationship to respective racks 22, the racks 23 extending from the left end of member 3 to approximately the middle thereof. Four pinions 24 (two of which are shown) are provided on opposite sides of cylinder 4 and engage confronting racks 22 and 23, as illustrated in FIG. 6. Each of the pinions 24 includes a bearing which is connected to the aforedescribed type of buckle-preventing device 9 by means of rods 25.

It is readily apparent that as the piston 5 of FIG. 6 is moved between its contracted and extended position for moving inner boom member 3 in a similar manner, the racks 23 are moved the same distance relative to stationary racks 22. In this manner, the pinions 24, through connecting rods 25, move device 9 one-half of this distance, in the extending or contracting direction of the boom 1, whereby the device is always positioned at an intermediate position between the free end of the extended piston-rod 5 and the cylinder 7.

In addition, when the pinions 24 are engaged with racks 22 provided on the outer circumference of the cylinder 7 at equally circumferentially spaced intervals (only racks 22 at upper and lower sides are shown in FIG. 6), the free end of the cylinder 7 is supported to the inner boom member 3 by the four pinions 24 so that the hydraulic cylinder unit 4 is given an increased resistivity against buckling thereof.

Although a normally postured hydraulic cylinder unit 4 is employed in the embodiment shown in FIG. 6, driving means for the buckle-preventing device 9 comprising racks and pinions can also be employed when an inverted or upside-down hydraulic cylinder unit is used, as, for example, shown in FIG. 7. In the embodiment shown in FIG. 7, a fixed rack 23 is secured on the inner longitudinal circumference of the inner boom member 3 and in confronting relationship with a rack 22 which is fixed at one end to the lower end of an outer boom member 2 at 26 and which extends into inner boom member 3 where it is attached to a roller 27 and rotatable within and along the lower end of the inner boom. While the roller 27 is shown attached to rack 22 and movable along the inner surface of boom member 3, it is to be understood that the roller may be fixed to the inner boom and positioned in rotatable engagement with the back of the rack 22 for serving the same function. In either case, the racks include a pinion 24 and rod connected to device 9 for moving device 9 in the same manner as discussed with regard to FIG. 6.

It is to be understood that driving means for the buckle-preventing device 9 comprising a combination of racks and pinions can also be employed to a multistage extensible boom having more than three boom members, as shown, for example, in FIG. 8. In FIG. 8, numeral 4 designates an additional hydraulic cylinder unit for a third boom member, numeral 9, designates an additional buckle-preventing device, 22 and 23 and 25 an additional connecting rod, respectively, all of which operate in the same manner as described with respect to FIGS. 6 and 7.

Each of embodiments of the present invention shown in FIGS. 6, 7 and 8 provides another driving means for the buckle-preventing device wherein said device is moved by bearing means for a pinion or pinions engaging with a pair of racks at the same time which racks are provided at definite portions on the cylinder of a hydraulic cylinder unit or a boom member carrying said cylinder and on a boom member carrying the piston-rod of said cylinder unit in face-to-face and parallel relation.

Turning to FIGS. 9 and 10, attention is directed to another embodiment of the present invention. In the extensible boom 1 shown in FIGS. 9 and 10, four equally circumferentially spaced rollers 28 covered with plastic tires such as gum tires are provided between and in engagement with the upper half portion of a cylinder 7 and the lower half portion of an inner boom member 3. The bearings 29 of rollers 28 are connected to a buckle-preventing device 9 by rods 30. This construction provides an advantage over the embodiments of FIGS. 7 and 8 in that there is no need for the aforementioned racks. In this extensible boom 1, when the piston-rod 5 is extended by an optional distance, the rollers are moved or rolled along the outer circumference of the cylinder 7 and the inner circumference of the boom member 3 so that the device 9 is moved with the rollers 28 by a distance one-half of extended distance of the piston-rod 5, whereby the device 9 is always positioned at an intermediate position between the free end portion of the extended piston-rod 5 and the lower end of the cylinder 7.

With the utilization of at least a pair of rollers 28 provided on the outer circumference of the cylinder 7, as is the case in the embodiment shown in FIGS. 9 and 10, and in the event of a buckling load being applied to the cylinder unit 4, at least one of the rollers 28 will always be in engagement with both the cylinder 7 and the inner boom member 3, whereby the movement of the device 9 toward the extending or contracting direction is guaranteed more surely than if only a single roller 28 were provided. However, when four rollers 28 are arranged equally circumferentially around the cylinder 7, as illustrated in FIG. 10, the free end portion of cylinder 7 is supported 'to the inner boom member 3 through these four rollers 28, for enhancing the resistivity against buckling of hydraulic cylinder unit 4 at the portion thereof where the rollers 28 are positioned.

FIG. 11 shows a modification of the embodiment shown in FIGS. 9 and 10. In this embodiment wherein an inverted or upside-down hydraulic cylinder unit 4 is arranged in an extensible boom 1, there is provided an additional plate member 31, one end of which is fixed at 32 to the lower end portion of an outer boom member 2 which is also connected to a piston-rod 5 of the cylinder unit 4, the plate member extending longitudinally into inner boom member 3, as illustrated in FIG. 1 1. Further, a pair of rollers 28 are positioned for rolling engagement between and to plate member 31 and an inner boom member 3, the bearings of rollers 28 being connected to a buckle-preventing device 9 by rods 30, so that device 9 can be moved with aid of the rolling action of the rollers 28 in the same manner as described in the embodiment shown in FIGS. 9 and 10.

FIG. 12 shows a modification of the rollers 28 used in the above embodiment of FIG. 11, which modification is directed towards reducing the diameters of such rollers as well as the need for rollers with accurately di mensioned diameters. In this preferred modification, a plurality of rollers 28,, 28 and 28 are provided and are rotatably mounted on bearing means 34 pivotally connected to the end of rod 30 at 33, the rollers operating in connection with one another. The rollers 28;, and 28 which pivot about intermediate roller 28 along with bearing means 34, are respectively forced against the inner boom member 3 and the plate member 31 with aid of a spring 35 provided in the bearing means 34, respectively. In this manner, the rollers 28, 28 and 28 provide the same function as a single roller 28 of FIG. 11.

It is to be understood that the driving means for the buckle-preventing device 9 used in the embodiment shown in FIG. 11 can also be applied to a multi-stage extensible boom having three or more boom members by providing the required numbers of plate members 31. In the three-stage extensible boom shown in FIGS. 13 and 14, plate members 31a and 31b are fixed at common ends thereof to respective boom members 1 and 2 at 32a and 32b and are supported at their free end portions, by the means shown in FIG. 14. That is, the plate member 31a is engaged between supporting rollers 36a and 36b rotatably mounted on onestage higher boom member 2 so as to keep the plate at a definite position. In addition, bearing means 34a provided with a roller 28a for moving buckle-preventing device 9a includes two rollers 37 and 37 positioned on opposite sides of roller 28a. Rollers 37 and 28a receive therebetween and engage opposite sides of the plate 1 member 31a while the rollers 37 and 28a receive therebetween and engage opposite sides of plate member 31b, whereby the plate members 31a and 31b are kept at definite positions which ensures the engagement of the roller 28a therewith. Furthermore, a roller 28b (FIG. 13) for moving a buckle-preventing device device wherein said device is moved by a roller (rollers) which is (are) slidably engaged with'the cylinder,

a boom member carrying the cylinder or a plate member fixed to a suitable portion of said boom member, and also a boom member carrying the piston-rod or a plate member fixed to a suitable portion of said boom member.

Attention is directed to FIGS. and 16 for a further embodiment of the present invention. In this embodiment, a buckle-preventing device 9 is provided with a rotary drum 40 the longitudinal shaft 39 of which is supported to the device 9 in a direction transverse of the movement of the device. Between the drum 40 and the extending and contracting boom members, there is provided a pair of wire ropes 45 and 46 common ends of which are fixed to an outer boom member 2 and the inner boom member 3, repsectively, at suitable positions 41 and 42 (in this case, at the lower end portions of said members) under a condition where the boom 1 is fully extended. The otherwise free common ends of ropes 45 and 46 are fixed to the drum 40 at 43 and 44, respectively. Further, between the drum 40 and the buckle-preventing device 9, there is provided a plate spring 47 the ends of which are fixed to the drum 40 at 48 and to the device 9 at 49, the plate spring forceably rotating .the drum 40 in such a direction that the ropes 45 and-46 will be wound up around the drum. When a piston-rod 5 is extended or contracted by an optional distance, the drum 40 is rotated with or against the force of the spring 47 so that the pair of wire ropes 45 and 46 are wound up or unwound under tension,

whereby the device 9 is moved by a half of the extended or contracted distance of the piston-rod 5. In this way, the device is always positioned at an intermediate position between the free end portion of the extended piston-rod 5 and the lower end of the cylinder 7.

Though an inverted or upside-down hydraulic cylinder unit 4 and a two stage extensible boom are shown in FIGS. 15 and 16, the driving means for the bucklepreventing device shown in FIGS. 15 and 16 can, of course, be applied to a multi-stage extensible boom having any number of boom members and provided with normally postured hydraulic cylinder units, as can now clearly be understood by those skilled in the art. Thus, this embodiment further provides compact driving means for the buckle-preventing device wherein said device is moved by a rotary drum which is rotatably mounted to the device and which is always forced in such a direction that wire ropes or chains will be wound up on said drum, the wire ropes or chains being fixed to the drum at common ends and to the relatively extending and contracting boom members.

In briefly summarizing the preferred embodiment of the present invention as illustrated in FIGS. 1 through 16, a device is provided for substantially eliminating buckling of a hydraulic cylinder unit which is used for driving an extensible boom. The buckle-preventing device is positioned around the piston-rod of the cylinder unit within one of the boom members and includes outwardly extending legs which slidably or rotatably engage the inner surface of the boom member for longitudinally moving the device therethrough. Various types of driving means (as illustrated in the various figures) are provided for moving the device along with the piston-rod (or cylinder component) such that the device is always positioned at substantially an intermediate point between the free end of the piston-rod and the end of cylinder of said cylinder unit. In this manner, the piston-rod is always supported with the bucklepreventing device at what is otherwise the critical buckling point whereby resistance against such buckling becomes effectively increased at any extended state of said rod. Therefore, according to the present invention, the weight of the hydraulic cylinder unit can be decreased correspondingly, so that an extensible boom with a longer scale can be obtained-without increasing substantial weight and cost.

The following are further detailed embodiments of driving means for the buckle-preventing device. One such embodiment is illustrated in FIGS. 17 and 18. In this embodiment, the buckle-preventing device 9 is spring-biased in the direction'of arrow A by a spring 50a provided betweensaid device 9 and a suitable portion of the cylinder 7. A rod member 52 having a stopper 51 at the uppermost portion is slidably inserted through the device and has its other end secured to the free end portion of piston-rod 5. In this manner, the device 9 is biased into engagement with the lower end of the cylinder 7, due to the force of the spring 50a, so long as the hydraulic cylinder unit 4 is in a contracted state or in an extended state with an extended distance equal to or shorter than a definite L, the effective movement of the device 9 in the outer boom member (FIG. 17) so that the span between the device 9 and the pivot 6 of the piston-rod is maintained at the aforementioned definite distance L. The distance L is preferably designed so as to correspond to one-half of the fully extended span of the piston-rod 5, so that the device 9 is positioned substantially intermediate the free end of the piston-rod and cylinder 7. Further, in the embodiment shown in FIGS. 17 and 18, there is provided a rod member 53 which has one end secured to the device 9, and the other end, including a stopper 54, slidably inserted into a bracket 55 fixedly mounted on the outer circumference of the cylinder 7. The rod member 53 serves for preventing the interval between the device 9 and the cylinder 7 from being enlarged more than the definite one L.

Though an inverted or upside-down hydraulic cylinder unit 4 is employed in the embodiment shown in FIGS. 17 and 18, driving means for the bucklepreventing device according to the present embodiment can also be applied to an extensible boom with a normally postured hydraulic cylinder unit 4, as can be understood with reference to FIG. 19. Further, although the device 9 and the cylinder 7 are connected through spring 50a and a rod member 52 is arranged between the device 9 and the free end of the piston-rod 5 so as to keep the interval between the device 9 and said free end substantially equal to a definite distance when the unit 4 is in an extended state (as illustrated in FIG. 19) the device 9 may be so constructed that the interval between the device and the cylinder 7 is maintained fixed (for example, a distance L) when unit 4 is in its most extended state. This is achieved by connecting the device 9 to the free end of the piston-rod 5 by means of a spring and by arranging the aforementioned type of a rod member between the device and the cylinder. Furthermore, even if the spring 50a shown in FIG. 17 is omitted, the device 9 will be positioned substantially intermediate the free end of piston-rod 5 and cylinder 7 when the cylinder unit is in its fully extended position as limited by rod members 52 and 53.

In addition, more than two buckle-preventing devices may be provided with a piston-rod, as illustrated in FIG. 20. In this embodiment, two buckle-preventing devices 9a and 9b, each of which is constructed in the same manner as buckle-preventing device 9, are provided about the piston-rod 5 of cylinder unit 4. These two devices 9a and 9b are connected to each other with a spring 50b. Further, a rod member 53a having an end stopper 54a connects the device 9a and the cylinder 7 while a rod member 52b having an end stopper 54b connects the device 9b and the free end of piston-rod 5 so that the respective intervals between the device 9a and the cylinder 7 and between the device 9b and the free end of the piston-rod 5 are maintained fixed when the hydraulic cylinder unit 4 is fully extended.

It is to be understood that the rod members 52, 53, 52b and 53a shown in FIGS. 17 through 20 may be substituted with wire ropes, chains or the like having definite length. Further, although springs 50a and 50b are of the pulling type as illustrated in FIGS. 17 through 20, compression springs may be used for the same purpose when they are arranged in parallel arrangements with the rod members so as to keep said springs straight.

Each of embodiments of the present invention shown in FIGS. 17 through 20 provides another driving means for the buckle-preventing device wherein said device is moved during the extension of a hydraulic cylinder unit by a spring or a rod member while keeping the interval between the device and the free end of the piston-rod or between the device and the cylinder fixed when the hydraulic cylinder is in an extended state.

A further embodiment of the present invention is illustrated in FIGS. 21, 22 and 23. In this embodiment, restraining means are provided between the bucklepreventing device 9 and an inner boom member (although said means may be provided between the device 9 and the cylinder 7) which includes a pair of substantially diametrically aligned hooks 59 pivotally mounted on brackets 56 and 57 and spring forced in the direction of arrow B by means of associated springs 58. Each hook 59 includes an arm portion having a roller 60 rotatably mounted thereto. Each hook 59 is biased in a position shown in FIG. 21 (the same position is illustrated by imaginary lines in FIG. 23) when its associated roller 60 is in rotatable engagement with the inner circumference of the outer boom member 2. However, each of the hooks 59 is forced into a restraining position, as illustrated by solid lines in FIG. 23, when its roller 60 engages an associated cam surface 61 provided on the inner circumference of the outer boom member 2.

As the hydraulic cylinder unit 4 is extended from its contracted state by a definite distance L, the hooks 59 engage pins 62 projecting from the legs 10 of the buckle-preventing device 9, as seen best in FIG. 22. However, when the cylinder unit 4 is extended by a distance longer than the distance L, each of the rollers 60 engages and rides on an associated cam surface 61 causing the hook 59 to move from its biased position to its restrained position for disengaging the hook from pin 62 and therefore disengaging the hook from device 9. In this manner, the device 9 is restrained at a definite position in the outer boom member 2, namely at a position apart from the free end of the piston-rod 5 by a definite distance L corresponding to substantially onehalf of the fully extended span of the piston-rod 5. The

device 9 is kept at this position with the aid of optional means such as frictional resistance provided between the device 9 and the inner circumference of the outer boom member 2 or between the device 9 and the piston-rod 5. When the cylinder 7 extends beyond this point, the hooks 59, of course, move back to their biased positions.

When the hydraulic cylinder unit 4 is contracted from its fully extended state, the rollers 60 again are brought into riding engagement with cam surfaces 60 whereby hooks 59 are driven to their restrained positions for receiving associated pins 62. Further movement of the unit 4 towards its contracted position causes the hooks to move to their biased positions and into engagement with the pins so that the device 9 is necessarily moved together with the cylinder 7 when the cylinder unit 4 is contracted still further. In addition, there is provided a little backlash between the device 9 and the cylinder 9 in said restraining state.

In the embodiment shown in FIGS. 21, 22 and 23, an inverted or upside-down hydrauIic cylinder unit 4 is arranged in an extensible boom 1. However, it is to be understood that driving means for the buckle-preventing device 9 according to this embodiment can also be applied to an extensible boom in which a normally postured hydraulic cylinder unit is arranged, as hereinafter detailed with reference to the embodiment shown in FIGS. 24 through 28. In this embodiment, there are provided particular restraining means which limit the buckle-preventing device 9 to a desired intermediate position on the extended piston-rod without relying upon the aforementioned frictional resistance, or the like.

Specifically, bracket 63 (FIG. 26) is secured to the top portion of the cylinder 7 upon which a first hook 64 is pivotally mounted at 65, the hook being spring biased in the direction of arrow C by spring means 69. A roller 66 is pivotally mounted on one end portion of first hook 64. While the first hook 64 is always restrained by a stopper 67 projecting from bracket 63, or a pin 68 projecting from device 9, the hook is moved into the reverse direction of arrow C and out of engagement with the pin 68 when the roller 66 rides on a cam 70 provided on the inner circumference of the inner boom member 3 at an intermediate position thereof.

Turning to FIG. 28, a second hook 72 is pivotally mounted to the inner circumference of the inner boom member 3, at a point 73 in approximately the middle of the boom member, the second hook 72 being spring forced or biased in the direction of arrow D by means of a spring 71. In its biased position, the second hook 72 is restrained by a pin 74 projecting from the inner boom member 3. The base end of a lever 75 is pivotally connected to the inner circumference of boom member 3 at 76, and a spring 77 is arranged between the other end of lever 75 and a suitable portion of the second hook 72. The second hood 72 is restrained with the force of spring 71 on the aforementioned pin 74 or a pin 78 projecting from the device 9 so as to be maintained at a definite position (FIG. 28). However, the hook 72 is moved in the reverse direction of arrow D, against the force of the spring 77, when the lever 75 is operated by a pin 79 projecting from the upper portion of the cylinder 7, as well as by an inclined plane 720 provided at the front portion of the hook 72 when the plane engages the pin 78.

Operationally, in the embodiment shown in FIGS. 24 through 28, the first hook 64 engages pin 68 so that the buckle-preventing device 9 is restrained at the upper end portion of the cylinder when the piston-rod is in its contracted state (FIG. 25). When the inner boom member 3 has been extended to an intermediate position, the first hook 64 leaves the pin 68 with the aid of the roller 66 and the cam 70, and the second hook 72 becomes automatically engaged with the pin 78 due to the engagement of the inclined plane 72a and pin. Thereafter, the device 9 is moved together with the piston-rod 5 during extension thereof, so that said device 9 is positioned at an intermediate position of the inner boom member 3 (FIG. 24). When the piston-rod 5 is contracted from the above state, the device 9 is moved together with the contraction of the piston-rod 5 and the in 68 is passed along the inclined plane 64a to engage the first hook 64. At the same time, the second hook 72 is moved into the reverse direction of arrow D, due to the operation of the lever 75 by the pin 79, and is released from engagement with the pin 78 so that the piston-rod 5 is further contracted to its original state. Therefore, the device 9 can be maintained substantially in the middle of piston-rod 5 when the latter is in its fully extended position.

In each of embodiments of the present invention, as shown in FIGS. 17 through 28, the buckle-preventing means can be constructed such that, when the pistonrod has been extended by a definite distance, the buckle-preventing device is moved to a predetermined point on the extended piston-rod, said piston-rod being supported at a suitable intermediate position by the buck- 5 le-preventing device which is slidably or rotatably engaged with the inner circumference of the outer or inner boom member, and the maximum span of the piston-rod between the device and the free end of the piston-rod and between the device and the cylinder always being kept shorter than said definite distance, for example, about a half of the length of the fully extended piston-rod, whereby resistance against buckling of the extended piston-rod is increased correspondingly. Thus, according to the present invention, the weight of such a hydraulic cylinder unit can be decreased in proportion to the increase in resistance against buckling of the piston-rod. In addition, when utilizing an inverted or upside-down hydraulic cylinder unit, the bucklepreventing device is kept in contact with the lower end of the cylinder until the cylinder unit has been extended to an intermediate state, as is the case in the embodiments shown in FIGS. 17 and 18 and FIGS. 21 through 23, namely when the device is moved only at the beginning period of extending operation and the closing period of contracting operation of the hydraulic cylinder unit. Therefore, the buckle-preventing device is moved before the piston-rod has a chance to buckle, so that the device can be moved in a substantially smooth manner. Furthermore, when an inverted or upside-down cylinder unit is employed, the cylinder of such unit can be pivoted or fixed to the inner boom member at the lower end portion of said member whereby to increase the resistance against buckling. This result is achieved with the disclosed embodiments by employment of various types of lost motion connections, as illustrated, which produce movement of the buckle-preventing means relative to the hydraulic cylinder only after the degree of extension of the piston rod exceeds a predetermined value.

In addition, although a two-stage extensible boom is illustrated in each of the embodiments shown in FIGS. 17 through 28, each feature particularly disclosed in said embodiments can also be applied, added or mutually interchanged for multi-stage extensible booms having optional numbers of telescopically extensible boom members and hydraulic cylinder units arranged between two adjacent boom members. Further, although a piston-rod is provided with only one bucklepreventing device in each of the embodiments, except for that embodiment shown in FIG. 20, each feature particularly disclosed in saidembodiment can be applied, so long as no contradiction results, to modified buckle-preventing means wherein two or more than two buckle-preventing devices are provided to a pistonrod, and each of said devices is brought to a controlled position when the piston-rod is extended.

Thus, the present invention is in no way limited to the embodiments particularly described hereinbefore, and it is to be understood that innumerable variations, applications, modifications and extensions of the basic principles involved may be made without departing from its scope. The invention is to be limited, therefore, only by the scope of the appended claims.

What is claimed is:

1. In an extensible boom of the type having at least one pair of telescopically extensible inner and outer boom members and at least one hydraulic power unit arranged between the ends of adjacent inner and outer boom members for producing relative movement between said boom members, said power unit including a hydraulic cylinder connected to one of said boom members, and a piston rod connected to the other of said boom members; buckle-preventing means for said piston rod, said buckle-preventing means comprising: a body portion slidably received on said piston rod of said power unit, said body portion having outwardly extending means movably engaged with the inner circumference of the boom member to which the end of the piston is connected, and driving means for moving said body portion through said boom in response to the extending and contracting operation of said hydraulic power unit, said driving means including means operably connecting said body portion to said adjacent boom members during only apart of said extending and contracting operation and moving said body portion continuously during said part of the extending and contracting operation of said unit at a rate same as the extending and contracting rate of said hydraulic power unit, whereby respective distances between said body portion and the free end of said piston rod and between said body portion and the end of said hydraulic cylinder are always kept shorter than predetermined ones, said last mentioned means includes a rod member connected at one end to the free end of said piston-rod, said rod member locking into engagement with said body portion when said piston-rod extends a predetermined distance, and spring means connected to said body portion and cylinder for biasing said body portion in a direction away from the free end of said piston-rod.

2. In an extensible boom of the type having at least one pair of telescopically extensible inner and outer boom members and at least one hydraulic power unit arranged between the ends of adjacent inner and outer boom members for producing relative movement between said boom members, said power unit including a hydraulic cylinder connected to one of said boom members, and a piston rod connected to the other of said boom members; buckle-preventing means for said piston rod, said buckle-preventing means comprising: a body portion slidably received on said piston rod of said power unit, said body portion having outwardly extending means movably engaged with the inner circumference of the boom member to which the end of the piston is connected, and driving means for moving said body portion through said boom in response to the extending and contracting operation of said hydraulic power unit, said driving means including means operably connecting said body portion to said adjacent boom members during only a part of said extending and contracting operation and moving said body portion continuously during said part of the extending and contracting operation of said unit at a rate same as the extending and contracting rate of said hydraulic power unit, whereby respective distances between said body portion and the free end of said piston rod and between said body portion and the end of said hydraulic cylinder are always kept shorter than predetermined ones, said last mentioned means including a rod member connected at one end to said cylinder, said rod member locking into engagement with said body portion when said cylinder extends a predetermined distance, and spring means for biasing said body portion in a direction away from said cylinder.

3. In an extensible boom of the type having at least one pair of telescopically extensible inner and outer boom members and at least one hydraulic power unit arranged between the ends of adjacent inner and outer boom members for producing relative movement be tween said boom members, said power unit including a hydraulic cylinder connected to one of said boom members, and a piston rod connected to the other of said boom members; buckle-preventing means for said piston rod, said buckle-preventing means comprising: a body portion slidably received on said piston rod of said power unit, said body portion having outwardly extending means movably engaged with the inner circumference of the boom member to which the end of the piston is connected, and driving means for moving said body portion through said boom in response to the extending and contracting operation of said hydraulic power unit, said driving means including means operably connecting said body portion to said adjacent boom members during only a part of said extending and contracting operation and moving said body portion continuously during said part of the extending and contracting operation of said unit at a rate same as the extending and contracting rate of said hydraulic power unit, whereby respective distances between said body portion and the free end of said piston rod and between said body portion and the end of said hydraulic cylinder are always kept shorter than predetermined ones, said last mentioned means including a pair of rod members, each of which is connected at one end to the piston-rod and cylinder respectively, said rod members locking into engagement with said body portion after the predetermined point of operation of said cylinder unit.

4. In an extensible boom of the type having at least one pair of telescopically extensible inner and outer boom members and at least one hydraulic power unit arranged between the ends of adjacent inner and outer boom members for producing relative movement between said boom members, said power unit including a hydraulic cylinder connected to one of said boom members, and a piston rod connected to the other of said boom members; buckle-preventing means for said piston rod, said buckle-preventing means comprising: a body portion slidably received on said piston rod of said power unit, said body portion having outwardly extending means movably engaged with the inner circumference of the boom member to which the end of the piston is connected, and driving means for moving said body portion through said boom in response to the extending and contracting operation of said hydraulic power unit, said driving means including means operably connecting said body portion to said adjacent boom members during only a part of said extending and contracting operation and moving said body portion continuously during said part of the extending and contracting operation of said unit at a rate same as the extending and contracting rate of said hydraulic power unit, whereby respective distances between said body portion and the free end of said piston rod and between said body portion and the end of said hydraulic cylinder are always kept shorter than predetermined ones, said last mentioned means including a pin member fixed to said body portion and hook means fixed with said cylinder and biased in engagement with said pin member, said hook means including an arm portion operated by a cam provided on the boom member supporting said piston-rod for disengaging said hook means and said body portion when said body portion has reached said predetermined point.

5. In an extensible boom of the type having at least one pair of telescopically extensible inner and outer boom members and at least one hydraulic power unit arranged between the ends of adjacent inner and outer boom members for producing relative movement between said boom members, said power unit including a hydraulic cylinder connected to one of said boom members, and a piston rod connected to the other of said boom members; buckle-preventing means for said piston rod, said buckle-preventing means comprising: a body portion slidably received on said piston rod of said power unit, said body portion having outwardly extending means movably engaged with the inner circumference of the boom member to which the end of the piston is connected, and driving means for moving said body portion through said boom in response to the extending and contracting operation of said hydraulic power unit, said driving means including lost motion connection means operably connecting said body portion to said adjacent boom members for movement relative to said hydraulic cylinder during only a part of said extending and contracting operation and moving said body portion continuously during said part of the extending and contracting operation of said unit at a rate same as the extending and contracting rate of said hydraulic power unit, whereby respective distances between said body portion and the free end of said piston rod and between said body portion and the end of said hydraulic cylinder are always kept shorter than predetermined ones, and said lost motion connection means comprises rod means having one end fixed relative to the free end of said piston rod and the opposite end slidably engaged with said body portion, said opposite end including stop means engageable with said body portion for maintaining said slidable connection, and biasing means tending to move said body portion toward said hydraulic cylinder, such that during extension of said free end of the piston rod relative to said cylinder, said rod will move relative to said body portion until said stop means is engaged with said body portion, said engagement producing movement of said body portion with said piston rod and relative to said cylinder upon the continued extension of said piston rod, said biasing means maintaining said engagement during retraction of said piston rod until the degree of extension of said rod is less than said predetermined value.

6. In an extensible boom of the type having at least one pair of telescopically extensible inner and outer boom members and at least one hydraulic power unit arranged between the ends of adjacent inner and outer boom members for producing relative movement between said boom members, said power unit including a hydraulic cylinder connected to one of said boom members, and a piston rod connected to the other of said boom members; buckle-preventing means for said piston rod, said buckle-preventing means comprising: a body portion slidably received on said piston rod of said power unit, said body portion having outwardly extending means movably engaged with the inner circumference of the boom member to which the end of the piston is connected, and driving means for moving said body portion through said boom in response to the extending and contracting operation of said hydraulic power unit, said driving means including lost motion connection means operably connecting said body portion to said adjacent boom members for movement relative to said hydraulic cylinder during only a part of said extending and contracting operation and moving said body portion continuously during said part of the extending and contracting operation of said unit at a rate same as the extending and contracting rate of said hydraulic power unit, whereby respective distances between said body portion and the free end of said piston rod and between said body portion and the end of said hydraulic cylinder are always kept shorter than predetermined ones, said buckle-preventing means comprises at least two body portions slidably received on said piston rod, each said body portion having outwardly extending means, spring means joining said body portions and tending to bias said body portions into closely spaced relation, said lost motion connection means comprising a first elongate member having one end fixed relative to the free end of said piston rod and being connected with a first one of said body portions while permitting relative movement between said free end and said body portion provided the degree of extension of said piston rod does not exceed the length of said elongate member, and a second elongate member having one end connected to the second one of said body portions with the opposite end connected to said hydraulic cylinder and limiting the amount of relative movement of said second body portion away from said cylinder to the length of said elongate member.

7. A combination as defined in claim 6 wherein said first and second elongate members comprise rod members and the respective connections to said first body portion and said cylinder are slidable connections.

8. In an extensible boom of the type having-at least one pair of telescopically extensible inner and outer boom members and at least one hydraulic power unit arranged between the ends of adjacent inner and outer boom members for producing relative movement between said boom members, said power unit including a hydraulic cylinder connected to one of said boom members, and a piston rod connected to the other of said boom members; buckle-preventing means for said piston rod, said buckle-preventing means comprising: a body portion slidably received on said piston rod of said power unit, said body portion having outwardly extending means movably engaged with the inner circumference of the boom member to which the end of the piston is connected, and driving means for moving said body portion through said boom in response to the extending and contracting operation of said hydraulic power unit, said driving means including lost motion connection means operably connecting said body portion to said adjacent boom members for movement relative to said hydraulic cylinder during only a part of said extending and contracting operation and moving said body portion continuously during said part of the extending and contracting operation of said unit at a rate same as the extending and contracting rate of said hydraulic power unit, whereby respective distances between said body portion and the free end of said piston rod and between said body portion and the end of said hydraulic cylinder are always kept shorter than predetermined ones, said lost motion connection includes elongate means having one end fixed relative to the free end of said piston rod and the opposite end connected to said body portion, said elongate means permitting relative movement between the free end of said piston rod and said body portion provided the degree of extension of said piston rod does not exceed the length of said elongate means, and spring means tending to bias said body portion toward said hydraulic cylinder.

9. In an extensible boom of the type having at least one pair of telescopically extensible inner and outer boom members and at least one hydraulic power unit arranged between the ends of adjacent inner and outer boom members for producing relative movement between said boom members, said power unit including a hydraulic cylinder connected to one of said boom members, and a piston rod connected to the other of said boom members; buckle-preventing means for said piston rod, said buckle-preventing means comprising: a body portion slidably received on said piston rod of said power unit, said body portion having outwardly extending means movably engaged with the inner circumference of the boom member to which the end of the piston is connected, and driving means for moving said body portion through said boom in response to the extending and contracting operation of said hydraulic power unit, said driving means including lost motion connection means operably connecting said body portion to said adjacent boom members for movement relative to said hydraulic cylinder during only a part of said extending and contracting operation and moving said body portion continuously during said part of the extending and contracting operation of said unit at a rate same as the extending and contracting rate of said hydraulic power unit, whereby respective distances between said body portion and the free end of said piston rod and between said body portion and the end of said hydraulic cylinder are always kept shorter than predetermined ones, said lost motion connection including elongate means in the form of a rod member having one end fixed relative to the free end of said piston rod and the opposite end connected to said body portion by a slidable connection whereby relative movement is permitted between the free end of said piston rod and said body portion provided the degree of extension of said rod does not exceed the length of said elongate means, said rod members having stop means on the end opposite said end fixed relative to said piston rod for engaging said body portion to produce said movement of the body portion relative to said cylinder.

10. In an extensible boom of the type having at least one pair of telescopically extensible inner and outer boom members and at least one hydraulic power unit arranged between the ends of adjacent inner and outer boom members for producing relative movement between said boom members, said power unit including a hydraulic cylinder connected to one of said boom members, and a piston rod connected to the other of said boom members; buckle-preventing means for said piston rod, said buckle-preventing means comprising: a body portion slidably received on said piston rod of said power unit, said body portion having outwardly extending means movably engaged with the inner circumference of the boom member to which the end of the piston is connected, and driving means for moving said body portion through said boom in response to the extending and contracting operation of said hydraulic power unit, said driving means including lost motion connection means operably connecting said body portion to said adjacent boom members for movement relative to said hydraulic cylinder during only a part of said extending and contracting operation and moving said body portion continuously during said part of the extending and contracting operation of said unit at a rate same as the extending and contracting rate of said hydraulic power unit, whereby respective distances between said body portion and the free end of said piston rod and between said body portion and the end of said hydraulic cylinder are always kept shorter than predetermined ones, said lost motion connecting means comprises, releasable gripping means fixedly disposed relative to the end of said cylinder from which said piston rod extend and being engageable with means on said body portion to maintain said buckle-preventing means in position adjacent said cylinder end, cam means moveable relative to said releasable gripping means during extension and contraction of said piston rod, said cam means being engageable with said releasable gripping means to operate same thereby releasing said gripping means after the degree of extension of said piston rod exceeds said predetermined value, and means for moving said body portion with said piston rod and relative to said cylinder upon release of said gripping means.

11. A combination as defined in claim 10 wherein said means for moving said body portion with said piston rod comprises a slidable connection mounting said body portion on said piston rod, which connection produces frictional engagement between said body portion and said rod permitting said body portion to move with said rod when unrestrained, but allowing for movement of said body portion relative to said rod when said body portion is restrained by said gripping means 12. A combination as defined in claim 10 wherein said means for moving said body portion with said piston rod comprises, second gripping means mounted a fixed distance from the free end of said rod, said second gripping means releasably engaging said body portion upon release of said first mentioned means to move said body portion relative to said cylinder.

13. In an extensible boom of the type having at least one pair of telescopically extensible inner and outer boom members and at least one hydraulic power unit arranged between the ends of adjacent inner and outer boom members for producing relative movement between said boom members, said power unit including a hydraulic cylinder connected to one of said boom members, and a piston rod connected to the other of said boom members; buckle-preventing means for said piston rod, said buckle-preventing means comprising: a body portion slidably received on said piston rod of said power unit, said body portion having outwardly extending means movably engaged with the inner circumference of the boom member to which the end of the piston is connected, and driving means for moving said body portion through said boom in response to the extending and contracting operation of said hydraulic power unit, said drive means including lost motion connection means comprising releasable catch means for maintaining said body portion in position closely adjacent said hydraulic cylinder during those portions of the extending and contracting operation wherein the degree of extension of said rod is less than a predetermined value, and cam means for effecting release of said catch means thereby permitting said body portion to move relative to said hydraulic cylinder, and said lost motion connection means effecting operable connection of said body portion to the adjacent boom member for movement relative to said hydraulic cylinder during those portions of the extending and contracting operation of said boom, when the degree of extension of the rod is greater than a predetermined distance. 

1. In an extensible boom of the type having at least one pair of telescopically extensible inner and outer boom members and at least one hydraulic power unit arranged between the ends of adjacent inner and outer boom members for producing relative movement between said boom members, said power unit including a hydraulic cylinder connected to one of said boom members, and a piston rod connected to the other of said boom members; bucklepreventing means for said piston rod, said buckle-preventing means comprising: a body portion slidably received on said piston rod of said power unit, said body portion having outwardly extending means movably engaged with the inner circumference of the boom member to which the end of the piston is connected, and driving means for moving said body portion through said boom in response to the extending and contracting operation of said hydraulic power unit, said driving means including means operably connecting said body portion to said adjacent boom members during only a part of said extending and contracting operation and moving said body portion continuously during said part of the extending and contracting operation of said unit at a rate same as the extending and contracting rate of said hydraulic power unit, whereby respective distances between said body portion and the free end of said piston rod and between said body portion and the end of said hydraulic cylinder are always kept shorter than predetermined ones, said last mentioned means includes a rod member connected at one end to the free end of said piston-rod, said rod member locking into engagement with said body portion when said piston-rod extends a predetermined distance, and spring means connected to said body portion and cylinder for biasing said body portion in a direction away from the free end of said piston-rod.
 2. In an extensible boom of the type having at least one pair of telEscopically extensible inner and outer boom members and at least one hydraulic power unit arranged between the ends of adjacent inner and outer boom members for producing relative movement between said boom members, said power unit including a hydraulic cylinder connected to one of said boom members, and a piston rod connected to the other of said boom members; buckle-preventing means for said piston rod, said buckle-preventing means comprising: a body portion slidably received on said piston rod of said power unit, said body portion having outwardly extending means movably engaged with the inner circumference of the boom member to which the end of the piston is connected, and driving means for moving said body portion through said boom in response to the extending and contracting operation of said hydraulic power unit, said driving means including means operably connecting said body portion to said adjacent boom members during only a part of said extending and contracting operation and moving said body portion continuously during said part of the extending and contracting operation of said unit at a rate same as the extending and contracting rate of said hydraulic power unit, whereby respective distances between said body portion and the free end of said piston rod and between said body portion and the end of said hydraulic cylinder are always kept shorter than predetermined ones, said last mentioned means including a rod member connected at one end to said cylinder, said rod member locking into engagement with said body portion when said cylinder extends a predetermined distance, and spring means for biasing said body portion in a direction away from said cylinder.
 3. In an extensible boom of the type having at least one pair of telescopically extensible inner and outer boom members and at least one hydraulic power unit arranged between the ends of adjacent inner and outer boom members for producing relative movement between said boom members, said power unit including a hydraulic cylinder connected to one of said boom members, and a piston rod connected to the other of said boom members; buckle-preventing means for said piston rod, said buckle-preventing means comprising: a body portion slidably received on said piston rod of said power unit, said body portion having outwardly extending means movably engaged with the inner circumference of the boom member to which the end of the piston is connected, and driving means for moving said body portion through said boom in response to the extending and contracting operation of said hydraulic power unit, said driving means including means operably connecting said body portion to said adjacent boom members during only a part of said extending and contracting operation and moving said body portion continuously during said part of the extending and contracting operation of said unit at a rate same as the extending and contracting rate of said hydraulic power unit, whereby respective distances between said body portion and the free end of said piston rod and between said body portion and the end of said hydraulic cylinder are always kept shorter than predetermined ones, said last mentioned means including a pair of rod members, each of which is connected at one end to the piston-rod and cylinder respectively, said rod members locking into engagement with said body portion after the predetermined point of operation of said cylinder unit.
 4. In an extensible boom of the type having at least one pair of telescopically extensible inner and outer boom members and at least one hydraulic power unit arranged between the ends of adjacent inner and outer boom members for producing relative movement between said boom members, said power unit including a hydraulic cylinder connected to one of said boom members, and a piston rod connected to the other of said boom members; buckle-preventing means for said piston rod, said buckle-preventing means comprising: a body portion slidably received on said piston rod of said power unit, said Body portion having outwardly extending means movably engaged with the inner circumference of the boom member to which the end of the piston is connected, and driving means for moving said body portion through said boom in response to the extending and contracting operation of said hydraulic power unit, said driving means including means operably connecting said body portion to said adjacent boom members during only a part of said extending and contracting operation and moving said body portion continuously during said part of the extending and contracting operation of said unit at a rate same as the extending and contracting rate of said hydraulic power unit, whereby respective distances between said body portion and the free end of said piston rod and between said body portion and the end of said hydraulic cylinder are always kept shorter than predetermined ones, said last mentioned means including a pin member fixed to said body portion and hook means fixed with said cylinder and biased in engagement with said pin member, said hook means including an arm portion operated by a cam provided on the boom member supporting said piston-rod for disengaging said hook means and said body portion when said body portion has reached said predetermined point.
 5. In an extensible boom of the type having at least one pair of telescopically extensible inner and outer boom members and at least one hydraulic power unit arranged between the ends of adjacent inner and outer boom members for producing relative movement between said boom members, said power unit including a hydraulic cylinder connected to one of said boom members, and a piston rod connected to the other of said boom members; buckle-preventing means for said piston rod, said buckle-preventing means comprising: a body portion slidably received on said piston rod of said power unit, said body portion having outwardly extending means movably engaged with the inner circumference of the boom member to which the end of the piston is connected, and driving means for moving said body portion through said boom in response to the extending and contracting operation of said hydraulic power unit, said driving means including lost motion connection means operably connecting said body portion to said adjacent boom members for movement relative to said hydraulic cylinder during only a part of said extending and contracting operation and moving said body portion continuously during said part of the extending and contracting operation of said unit at a rate same as the extending and contracting rate of said hydraulic power unit, whereby respective distances between said body portion and the free end of said piston rod and between said body portion and the end of said hydraulic cylinder are always kept shorter than predetermined ones, and said lost motion connection means comprises rod means having one end fixed relative to the free end of said piston rod and the opposite end slidably engaged with said body portion, said opposite end including stop means engageable with said body portion for maintaining said slidable connection, and biasing means tending to move said body portion toward said hydraulic cylinder, such that during extension of said free end of the piston rod relative to said cylinder, said rod will move relative to said body portion until said stop means is engaged with said body portion, said engagement producing movement of said body portion with said piston rod and relative to said cylinder upon the continued extension of said piston rod, said biasing means maintaining said engagement during retraction of said piston rod until the degree of extension of said rod is less than said predetermined value.
 6. In an extensible boom of the type having at least one pair of telescopically extensible inner and outer boom members and at least one hydraulic power unit arranged between the ends of adjacent inner and outer boom members for producing relative movement between said boom members, said power unit including a hydraulic cylinder connected to one of said boom members, and a piston rod connected to the other of said boom members; buckle-preventing means for said piston rod, said buckle-preventing means comprising: a body portion slidably received on said piston rod of said power unit, said body portion having outwardly extending means movably engaged with the inner circumference of the boom member to which the end of the piston is connected, and driving means for moving said body portion through said boom in response to the extending and contracting operation of said hydraulic power unit, said driving means including lost motion connection means operably connecting said body portion to said adjacent boom members for movement relative to said hydraulic cylinder during only a part of said extending and contracting operation and moving said body portion continuously during said part of the extending and contracting operation of said unit at a rate same as the extending and contracting rate of said hydraulic power unit, whereby respective distances between said body portion and the free end of said piston rod and between said body portion and the end of said hydraulic cylinder are always kept shorter than predetermined ones, said buckle-preventing means comprises at least two body portions slidably received on said piston rod, each said body portion having outwardly extending means, spring means joining said body portions and tending to bias said body portions into closely spaced relation, said lost motion connection means comprising a first elongate member having one end fixed relative to the free end of said piston rod and being connected with a first one of said body portions while permitting relative movement between said free end and said body portion provided the degree of extension of said piston rod does not exceed the length of said elongate member, and a second elongate member having one end connected to the second one of said body portions with the opposite end connected to said hydraulic cylinder and limiting the amount of relative movement of said second body portion away from said cylinder to the length of said elongate member.
 7. A combination as defined in claim 6 wherein said first and second elongate members comprise rod members and the respective connections to said first body portion and said cylinder are slidable connections.
 8. In an extensible boom of the type having at least one pair of telescopically extensible inner and outer boom members and at least one hydraulic power unit arranged between the ends of adjacent inner and outer boom members for producing relative movement between said boom members, said power unit including a hydraulic cylinder connected to one of said boom members, and a piston rod connected to the other of said boom members; buckle-preventing means for said piston rod, said buckle-preventing means comprising: a body portion slidably received on said piston rod of said power unit, said body portion having outwardly extending means movably engaged with the inner circumference of the boom member to which the end of the piston is connected, and driving means for moving said body portion through said boom in response to the extending and contracting operation of said hydraulic power unit, said driving means including lost motion connection means operably connecting said body portion to said adjacent boom members for movement relative to said hydraulic cylinder during only a part of said extending and contracting operation and moving said body portion continuously during said part of the extending and contracting operation of said unit at a rate same as the extending and contracting rate of said hydraulic power unit, whereby respective distances between said body portion and the free end of said piston rod and between said body portion and the end of said hydraulic cylinder are always kept shorter than predetermined ones, said lost motion connection includes elongate means having one end fixed relative to the free end of said piston rod and the opposIte end connected to said body portion, said elongate means permitting relative movement between the free end of said piston rod and said body portion provided the degree of extension of said piston rod does not exceed the length of said elongate means, and spring means tending to bias said body portion toward said hydraulic cylinder.
 9. In an extensible boom of the type having at least one pair of telescopically extensible inner and outer boom members and at least one hydraulic power unit arranged between the ends of adjacent inner and outer boom members for producing relative movement between said boom members, said power unit including a hydraulic cylinder connected to one of said boom members, and a piston rod connected to the other of said boom members; buckle-preventing means for said piston rod, said buckle-preventing means comprising: a body portion slidably received on said piston rod of said power unit, said body portion having outwardly extending means movably engaged with the inner circumference of the boom member to which the end of the piston is connected, and driving means for moving said body portion through said boom in response to the extending and contracting operation of said hydraulic power unit, said driving means including lost motion connection means operably connecting said body portion to said adjacent boom members for movement relative to said hydraulic cylinder during only a part of said extending and contracting operation and moving said body portion continuously during said part of the extending and contracting operation of said unit at a rate same as the extending and contracting rate of said hydraulic power unit, whereby respective distances between said body portion and the free end of said piston rod and between said body portion and the end of said hydraulic cylinder are always kept shorter than predetermined ones, said lost motion connection including elongate means in the form of a rod member having one end fixed relative to the free end of said piston rod and the opposite end connected to said body portion by a slidable connection whereby relative movement is permitted between the free end of said piston rod and said body portion provided the degree of extension of said rod does not exceed the length of said elongate means, said rod members having stop means on the end opposite said end fixed relative to said piston rod for engaging said body portion to produce said movement of the body portion relative to said cylinder.
 10. In an extensible boom of the type having at least one pair of telescopically extensible inner and outer boom members and at least one hydraulic power unit arranged between the ends of adjacent inner and outer boom members for producing relative movement between said boom members, said power unit including a hydraulic cylinder connected to one of said boom members, and a piston rod connected to the other of said boom members; buckle-preventing means for said piston rod, said buckle-preventing means comprising: a body portion slidably received on said piston rod of said power unit, said body portion having outwardly extending means movably engaged with the inner circumference of the boom member to which the end of the piston is connected, and driving means for moving said body portion through said boom in response to the extending and contracting operation of said hydraulic power unit, said driving means including lost motion connection means operably connecting said body portion to said adjacent boom members for movement relative to said hydraulic cylinder during only a part of said extending and contracting operation and moving said body portion continuously during said part of the extending and contracting operation of said unit at a rate same as the extending and contracting rate of said hydraulic power unit, whereby respective distances between said body portion and the free end of said piston rod and between said body portion and the end of said hydraulic cylinder are always kept shorter than predetermined Ones, said lost motion connecting means comprises, releasable gripping means fixedly disposed relative to the end of said cylinder from which said piston rod extend and being engageable with means on said body portion to maintain said buckle-preventing means in position adjacent said cylinder end, cam means moveable relative to said releasable gripping means during extension and contraction of said piston rod, said cam means being engageable with said releasable gripping means to operate same thereby releasing said gripping means after the degree of extension of said piston rod exceeds said predetermined value, and means for moving said body portion with said piston rod and relative to said cylinder upon release of said gripping means.
 11. A combination as defined in claim 10 wherein said means for moving said body portion with said piston rod comprises a slidable connection mounting said body portion on said piston rod, which connection produces frictional engagement between said body portion and said rod permitting said body portion to move with said rod when unrestrained, but allowing for movement of said body portion relative to said rod when said body portion is restrained by said gripping means.
 12. A combination as defined in claim 10 wherein said means for moving said body portion with said piston rod comprises, second gripping means mounted a fixed distance from the free end of said rod, said second gripping means releasably engaging said body portion upon release of said first mentioned means to move said body portion relative to said cylinder.
 13. In an extensible boom of the type having at least one pair of telescopically extensible inner and outer boom members and at least one hydraulic power unit arranged between the ends of adjacent inner and outer boom members for producing relative movement between said boom members, said power unit including a hydraulic cylinder connected to one of said boom members, and a piston rod connected to the other of said boom members; buckle-preventing means for said piston rod, said buckle-preventing means comprising: a body portion slidably received on said piston rod of said power unit, said body portion having outwardly extending means movably engaged with the inner circumference of the boom member to which the end of the piston is connected, and driving means for moving said body portion through said boom in response to the extending and contracting operation of said hydraulic power unit, said drive means including lost motion connection means comprising releasable catch means for maintaining said body portion in position closely adjacent said hydraulic cylinder during those portions of the extending and contracting operation wherein the degree of extension of said rod is less than a predetermined value, and cam means for effecting release of said catch means thereby permitting said body portion to move relative to said hydraulic cylinder, and said lost motion connection means effecting operable connection of said body portion to the adjacent boom member for movement relative to said hydraulic cylinder during those portions of the extending and contracting operation of said boom, when the degree of extension of the rod is greater than a predetermined distance. 